1. Field of the Invention
The invention concerns a circuit arrangement for supplying electrode melting furnaces and, in particular, melting furnaces for electric slag remelting of metals, from a single or multi-phase network of the local supply system.
2. Description of the Prior Art
Melting furnaces of this kind for metal refining and measures to supply them are known from the German Auslegeschrift 1,917,494. The latter reference suggests that optimal metallurgical properties for a melting furnace can be realized by feeding the furnace with a frequency substantially lower than the line frequency of between about 5 and 10 Hz by means of a controlled converter arrangement having a variable frequency and conduction time.
The present invention involves a converter arrangement, provided between a supply network and a furnace installation, in which the conduction times and the waveform of the positive and negative half-waves of the supply current for the furnace, drawn from the converter, are optimally adapted to the metallurgical requirements, and in which the intervals between the aforesaid two half-waves are adapted to the properties (recovery protection times) of the thyristor groups associated with the converter. In particular, the invention is concerned with a problem frequently occurring with electrode melting-furnace supplies employing such a converter arrangement, namely, the problem of preventing the valuable thyristors of the converter and the electrical parts of the furnace from being overloaded as a result of the so called "running-up" of the output voltage of the current regular associated with the converter. Since such a current regulator is designed to increase its output voltage up to a maximum when the current value of the furnace is less than a predetermined current reference value i.e., to exhibit so called "PI behavior", "running-up" of the output voltage typically occurs during a lifting of an electrode from the melt or during one of the aforementioned currentless intervals provided for the recovery protection of the thyristors.
Ideally, the current waveform derived from the converter arrangement of an electrode melting furnace supply system should have a frequency that can be adjusted continuously, i.e., without jumps, within a frequency range of about 3 to 10 Hz and should have approximately a rectangular shape for each half-wave. Moreover because of the protection recovery time of the thyristors, the waveform should have a currentless interval between these half-waves which should be at least equal to, but preferably longer than, such protection time. As a result of the finite lead inductances of the furnace terminals, on the one hand, and of the aforementioned PI behavior of the current regulator of the supply system, on the other hand, the actual current waveform produced by the converter deviates from the desired rectangular shape, with a nonlinear rise and decay occurring in each half-wave. While efforts to increase the initial slope of the waveform after each zero interval have presented difficulties, since "overshoot" beyond the desired current value must be prevented in each half-wave at all costs, making the trailing edge of the half-wave current waveform steeper is desirable and is possible through the action of the appropriately designed command stage which is clock-controlled and acts on the control unit feeding the converter.